The present invention relates to a Voltage Source Converter having at least one phase leg connecting to opposite poles of a direct voltage side of the converter and comprising a series connection of switching cells, each said switching cell having at least two current paths between the terminals thereof with a first current path formed by one or more first semiconductor assemblies connected in series and having each a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith, and a second path including a series connection of on one hand at least one second semiconductor assembly having a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith and on the other at least one energy storing capacitor, a mid point of said series connection of switching cells forming a phase output being configured to be connected to an alternating voltage side of the converter, each said switching cell being configured to obtain two switching states by control of said semiconductor devices of each switching cell, namely a first switching state in which said first path is in a non-conducting state and the voltage across said at least one energy storing capacitor is applied across the terminals of the switching cell, and a second switching state, in which said first path is closed and a zero voltage is applied across the terminals of the switching cell, for obtaining a determined alternating voltage on said phase output.
Such converters with any number of said phase legs are comprised, but they have normally three such phase legs for having a three phase alternating voltage on the alternating voltage side thereof.
A Voltage Source Converter of this type may be used in all kinds of situations, in which direct voltage is to be converted into alternating voltage and conversely, in which examples of such uses are in stations of HVDC-plants (High Voltage Direct Current), in which direct voltage is normally converted into a three-phase alternating voltage or conversely, or in so-called back-to-back stations in which alternating voltage is firstly converted into direct voltage and this is then converted into alternating voltage, as well as in SVCs (Static Var Compensator), in which the direct voltage side consists of capacitors hanging freely. However, the present invention is not restricted to these applications, but other applications are also conceivable, such as in different types of drive systems for machines, vehicles etc.
A Voltage Source Converter of this type is known through for example DE 101 03 031 A1 and WO 2007/023064 A1 and is as disclosed there normally called a multi-cell converter or M2LC. Reference is made to these publications for the functioning of a converter of this type. Said switching cells of the converter may have other appearances than those shown in said publications, and it is for instance possible that each switching cell has more than one said energy storing capacitor, as long as it is possible to control the switching cell to be switched between the two states mentioned in the introduction.
Another Voltage Source Converter of this type is known through U.S. Pat. No. 5,642,275 used in a Static Var Compensator, in which the switching cells have a different appearance in the form of so-called full bridges.
The present invention is primarily, but not exclusively, directed to such Voltage Source Converters configured to transmit high powers, and the case of transmitting high powers will for this reason mainly be discussed hereinafter for illuminating but not in any way restricting the invention thereto. When such a Voltage Source Converter is used to transmit high powers this also means that high voltages are handled, and the voltage of the direct voltage side of the converter is determined by the voltages across said energy storing capacitors of the switching cells. This means that a comparatively high number of such switching cells are to be connected in series for a high number of semiconductor devices, i.e. said semiconductor assemeblies, are to be connected in series in each said switching cell, and a Voltage Source Converter of this type is particularly interesting when the number of the switching cells in said phase leg is comparatively high. A high number of such switching cells connected in series means that it will be possible to control these switching cells to change between said first and second switching state and by that already at said phase output obtain an alternating voltage being very close to a sinusoidal voltage. This may be obtained already by means of substantially lower switching frequencies than typically used in known Voltage Source Converters of the type shown in FIG. 1 in DE 101 03 031 A1 having switching cells with at least one semiconductor device of turn-off type and at least one free-wheeling diode connected in anti-parallel therewith. This makes it possible to obtain substantially lower losses and also considerably reduces problems of filtering and harmonic currents and radio interferences, so that equipment therefor may be less costly.
In a Voltage Source Converter of this type, where several switching cells may be connected in series in order to handle high voltages, reliability may be reduced since a failure in a single switching cell or semiconductor assembly thereof may jeopardize the operation of the entire converter. WO 2007/023064 discloses a solution to this problem by achieving redundancy. This is made by short-circuiting a failing switching cell by the arrangement of a by-pass switch. However, this puts high demands on the reliability of the means, i.e. the switch, used for short-circuiting the switching cell and it also requires provision of a reliable control of said means.